Change single voice PDF page to A4 landscape

Added new parameters ```PrintParam.DEFAULT_A4PartLandscape``` so the voices are rendered in the PDF on A4 landscape pages.

Using the ```PrintParam``` is also a better way to solve the problem addressed in pull request #7.
it's not obvious from the scant documentation and poor UI, but the difference between these, by the way, is that the "print preview" view adds line breaks in order to space things fairly evenly, whereas "view parts" is supposed to display the parts with the line breaks of the currently-selected version/source, giving something closer to the source's layout. the latter view at some point seems to have stopped working as it should, and i should fix that, but the solution in any case isn't to make it silently the same as the other option, that's just even more confusing.